1. Field of the Invention
This invention relates to a vertical deflection circuit used in a display device employing a cathode-ray tube (CRT) that displays an image by scanning an electron beam. The invention also relates to a processor used in this vertical deflection circuit and to a CRT display using the aforementioned vertical deflection circuit.
2. Background Art
CRTs having electron guns that emit an electron beam scanned vertically and horizontally in two dimensions to display an image on a viewing screen have been heretofore known. In this kind of CRT display, an electron beam is deflected in two dimensions by magnetic forces. Thus, substantially horizontal scanning lines are drawn in turn from the top end of the viewing screen toward the bottom end. In this way, the CRT accomplishes a display of an image.
The magnetic forces for deflecting the electron beam are normally offered from vertical and horizontal deflection coils. These coils are mounted in the path of the electron beam. Electric currents are supplied to the coils from vertical and horizontal deflection circuits. Owing to the supply of the currents, the coils can generate magnetic fields necessary to deflect the electron beam vertically and horizontally.
FIG. 18 is a diagram illustrating an electric current (hereinafter referred to as the xe2x80x9cvertical deflection currentxe2x80x9d) supplied from the vertical deflection circuit to the vertical deflection coils. In FIG. 18, the horizontal axis indicates time, while the vertical axis indicates the value of the vertical deflection current. Point A indicates the value of the vertical deflection current at the top end of the viewing screen. Point B denotes the value of the vertical deflection current at the bottom end of the viewing screen.
As mentioned previously, the scanning lines are successively drawn from the top end toward the bottom end of the viewing screen. Therefore, the vertical deflection current supplied to the vertical deflection coils is so set as to vary linearly from the top end of the viewing screen toward the bottom end to vary the vertical deflection angle of the electron beam in substantially uniform angular increments. To return the electron beam to the top end of the viewing screen after reaching the bottom end of the viewing screen, the vertical deflection current is so set as to return to its original value during a short retrace period and again start to vary linearly.
Some of vertical deflection circuits for generating such vertical deflection currents produce vertical deflection currents digitally. This kind of vertical deflection circuit normally generates a digital current signal by a digital value-generating portion corresponding to some functions of a digital signal processor (DSP). A vertical deflection current corresponding to the generated digital current signal is produced. The digital current signal is generated during each main period in which the vertical deflection angle of the electron beam is varied, and has one digital current value.
The vertical deflection circuit is described in further detail. This vertical deflection circuit includes a D/A (digital-to-analog) converter, a low-pass filter (LPF), and a driver circuit, in addition to the digital signal value-generating portion described above. The D/A converter converts a digital current signal into an analog current signal. In this case, the analog current signal has an analog current value that shows a 1:1 correspondence to one digital current value forming the digital current signal. The analog current signal is supplied to the driver circuit via the LPF. The driver circuit supplies a current corresponding to the aforementioned analog current value as a vertical deflection current to the vertical deflection coils.
However, this vertical deflection circuit has the disadvantage that it may not be able to convert the digital current signal into an analog current signal of a desired amplitude due to distortion in the linearity of the input/output characteristics of the D/A converter. In this case, the angular increment of the deflection angle of the electron beam varies from location to location. As a result, spacing between the scanning lines becomes nonuniform, giving rise to fringes of varying shade on the viewing screen.
More specifically, the vertical deflection circuit is intended to vary the vertical deflection current linearly over the whole period for displaying a frame of image. That is, the vertical deflection circuit linearly varies the deflection angle of the electron beam to draw scanning lines at regular intervals, thus displaying an image at a uniform density.
However, where the linearity of the input/output characteristics of the D/A converter has distortion, the vertical deflection current varies in unequal increments. As a result, the vertical deflection angle of the electron beam varies in unequal angular increments In particular, the input/output characteristics of the D/A converter are normally distorted as indicated by the solid line in FIG. 19(a) from ideal characteristics indicated by the broken line. That is, even if the digital current signal is varied in uniform increments, the analog current signal produced from the D/A converter varies in nonuniform increments.
In this case, in locations where the analog current signal varies in larger increments, the deflection angle of the electron beam varies in greater increments, resulting in lower scanning line densities.Consequently, the viewing screen becomes darker.
On the other hand, in locations where the analog current is varied in smaller increments, the deflection angle of the electron beam varies in smaller increments, resulting in higher scanning line densities. The result is that the viewing screen becomes brighter. Hence, the viewing screen contains both locations 50 where the scanning lines density is lower and hence are darker and locations 60 where the density is higher and hence are brighter as shown in FIG. 19(b). This produces a fringe pattern.
Accordingly, it is an object of the present invention to provide a vertical scanning circuit that solves the foregoing technical problems and is capable of producing a vertical scanning current which varies linearly if the linearity of the input/output characteristics of the D/A converter has distortion.
It is another object of the invention to provide a processor used in the vertical deflection circuit described above.
It is a further object of the invention to provide a CRT display that uses the above-described vertical deflection circuit to thereby make uniform the scanning line density.
This invention is intended to achieve the aforementioned objects and pertains to a vertical deflection circuit for generating a vertical deflection current supplied to vertical deflection coils that generate a magnetic field necessary to deflect an electron beam vertically. That is, a vertical deflection circuit in accordance with this invention comprises a digital signal value-generating portion for producing a digital current signal, a D/A converter for converting the digital current signal produced by this digital value-generating circuit into an analog current signal, a filter for extracting the average value of the analog current signal produced from the D/A converter, and a driver circuit for supplying a vertical deflection current corresponding to the average value of the analog current signal extracted by the filter to the vertical deflection coils described above. This digital value-generating portion produces a digital current signal that shows plural different digtal values during a predetermined each main period which a vertical deflection angle of the electron beam is varied. The digital current values determine vertical deflection current values.
In this structure, the digital current signal showing plural different digital current values during each main period is produced. The average value of the analog current signal corresponding to the digital current signal is taken as the reference for the vertical deflection current. Accordingly, if the linearity of the input/output characteristics of the D/A converter has distortion, the vertical deflection current can be varied in substantially uniform increments. Therefore, the vertical deflection angle of the electron beam can be varied in substantially uniform angular increments. This prevents the scanning line density from becoming nonuniform. In consequence, a high-quality image can be produced on the viewing screen.
The above-described digital value-generating portion can be one function of a processor used in a vertical deflection circuit that supplies a vertical deflection current to vertical deflection coils after the average value of the analog current signal converted from the digital current signal by the D/A converter is extracted by a filter, the vertical deflection current corresponding to the average value. That is, this processor has a digital value-generating means that generates a digital current signal having plural different digital current values defining vertical deflection current values during a predetermined each main period in which the vertical deflection angle of the electron beam is varied and delivers this digital current signal.
In this structure, the digital current signal is generated in software and so it is easy to cope with the situation whatever input/output characteristics does the D/A converter have, the D/A converter being used as a part of the vertical deflection circuit. Therefore, excellent generality can be given to the vertical deflection circuit.
The present inventors have found that the method of selection of cells forming the D/A converter is one cause of generation of distortion in the linearity of the input/output characteristics of the D/A converter.
In particular, in the prior art D/A converter, where plural cells are selected, they may be spaced from each other. Cells located close to each other show similar characteristics. However, cells remotely spaced from each other exhibit different characteristics. In this way, cells normally are not uniform in characteristics. Accordingly, if cells remote from each other are selected, the linearity of the input/output characteristics of the D/A converter is distorted.
Accordingly, it is conceivable that this invention for achieving the aforementioned objects makes use of a D/A converter including plural cells arranged in two dimensions and turned on and off and a cell-selecting means for selecting only adjacent cells when a necessary number of cells are turned on based on the digital current value contained in the digital current signal.
In this structure, only mutually adjacent cells are selected and so distortion in the characteristics can be suppressed unlike in the prior art technique in which cells physically remotely spaced from each other have produced distortion in the characteristics. Therefore, the input/output characteristics of the D/A converter can be stabilized. Consequently, the vertical deflection current can be varied in substantially uniform increments. This assures that the vertical deflection angle of the electron beam is varied in substantially uniform angular increments. This prevents the scanning line density from becoming nonuniform. Hence, a high-quality image can be produced on the viewing screen.
In addition, the aforementioned vertical deflection circuit can be used in a CRT display. In particular, this CRT display comprises an electron gun for emitting an electron beam, vertical deflection coils for producing a vertical magnetic field necessary to deflect the electron beam vertically, the aforementioned vertical deflection circuit for producing a vertical deflection current necessary to produce the vertical magnetic field required to deflect the electron beam vertically, horizontal deflection coils for producing a horizontal magnetic field necessary to deflect the electron beam horizontally, and a display portion on which scanning lines are drawn by the electron beam whose direction of travel is deflected in two dimensions by the magnetic fields produced by the vertical deflection coils and the horizontal vertical deflection coils.
In this structure, the scanning line density can be made uniform. Consequently, a high-quality image can be produced on the viewing screen.